Family of six you may end up larger starting trailer than you are anticipating. And if you already have plans for a much larger 5th wheel in the future, then go for the bigger truck now. For that 40' future 5th wheel go ahead and get a dually. You will need it. Otherwise you definitely will be limited in your choices.
New trucks are rated for very high payloads and tow ratings now. But still, you need to look toward the future. Check weights of those larger 5th wheels and see just how much truck you are going to need. Don't buy too little truck now and take a loss on it to move up to a larger one in the not too distant future.